In measuring devices of process, and automation, technology, relays are sometimes used, in order to signal the presence of switch states or operating states.
A basic requirement for the development of electronic circuits is always a good power balance, i.e. a power loss that is as low as possible. In the case of operation of a relay, a relatively large amount of power loss can be saved. The relay requires, indeed, a pull-in voltage, which corresponds to the nominal voltage of the relay; however, to hold the relay in the pull-in state, a much smaller voltage, the so called holding voltage, is required. This is clearly shown on the basis of a simple example. Let us say we have a relay with a nominal voltage of 24 V and a resistance of 1600Ω. In order to bring the relay securely into the pulled-in state, the relay is provided with 24 V. From that, an electrical current of 24 V/1600Ω=15 mA results. Associated with that is a power loss of P=15 mA*24 V=360 mW. In order to lessen this power loss, a so called holding circuit is often used, in the case of which the electrical current is lessened to the holding current. The high voltage of 24 V is kept, however, so that this voltage still negatively affects other circuit parts as regards the power balance. This is particularly the case with linear regulators connected thereafter.